sh: ftrace: Fix missing parenthesis in WARN_ON()

sh: ftrace: Fix missing parenthesis in WARN_ON()

Adding a function inside a WARN_ON() didn't close the WARN_ON parathesis.

Link: http://lkml.kernel.org/r/201901020958.28Mzbs0O%fengguang.wu@intel.com
Cc: linux-sh@vger.kernel.org
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 9305800ce42e ("sh: ftrace: Use ftrace_graph_get_ret_stack() instead of curr_ret_stack")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>